This invention concerns an electrochemical sensor having an immobilized enzyme membrane and, more specifically, it relates to an electrochemical sensor suitable to the measurement of ingredients contained in living body fluids by utilizing enzyme catalyzed reactions.
It has become more and more important in doctor's diagnosis and therapy for patients, as well as in various other fields to measure or monitor the concentration of various substances such as urea, glucose and amino acids contained in living body fluids.
In view of the above, various types of enzyme electrodes comprising various ion selective electrodes or gas electrodes and immobilized enzymes in combination have been developed in order to conduct such measurement with ease and rapidity. The analysis using these enzyme electrodes has many advantages as compared with the conventional colorimetric method in that the structure of the measuring device is simplified, the time required for the analysis is shorter and the analysis can be made on a smaller amount of specimen.
For instance, a urea electrode comprises a combination of an ammonium ion electrode and an immobilized urease membrane, with which the following reactions are carried out. ##STR1##
In this enzyme electrode, urea contained in the blood is decomposed by urease, and ammonia as the decomposition product (hereinafter referred to as NH.sub.3) is detected by a NH.sub.3 gas electrode or detected, after being converted into ammonium ions (hereinafter referred to as NH.sub.4.sup.+) by a NH.sub.4.sup.+ selective electrode, in which the urea concentration in the blood indispensable for the diagnosis of nephrosis or the like can be calculated based on the potential change at the electrode.
An example of the urea electrode is disclosed, for instance, in Analytical Chemistry; vol. 45, No. 2 p 417 (1973). In this example, the ammonium ion selective electrode has on its surface a thin immobilized urease layer. In such an enzyme electrode, while the NH.sub.4.sup.+ selective membrane and the immobilized urease membrane are merely overlapped with each other, good bondability is not obtainable between the two membranes and, further, high responsivity can not be obtained.
Since the conventional enzyme electrode comprising the combination of the ion selective electrode and the immobilized enzyme membrane has a drawback in the response time as described above and although it can be used in combination with a batch type measuring cell in which no severe conditions are required for the analysis cycle, it can not be applied to the rapid and continuous measurement of specific chemical substances contained in living body fluids while being incorporated in a flow type measuring cell.